With the recent development of portable devices such as personal computers and cellular phones, there is an increasing demand for batteries used as the power source for such devices. In such applications, batteries are required to operate at room temperature and provide high energy density and excellent cycle characteristics. To meet such requirements, silicon (Si) or tin (Sn), and oxides or alloys thereof are viewed as promising negative electrode active materials that can offer significantly high capacities.
However, when such a material absorbs lithium, its crystal structure changes, so that its volume increases. A large volume change in an active material on charge/discharge results in, for example, a poor contact between the active material and a current collector and therefore a reduction in charge/discharge cycle life.
Japanese Patent No. 3702224 (hereinafter “Patent Document 1”) proposes forming an amorphous silicon thin film on copper foil by vapor deposition or sputtering. Patent Document 1 states that due to the diffusion of copper in the silicon thin film, the silicon thin film is firmly bonded to the copper foil, and therefore that even an expansion of silicon does not result in a degradation of current collecting performance.
However, since the diffusion coefficient of copper in silicon is high, copper may excessively diffuse into the silicon thin film. As a result, the copper foil becomes brittle, and further, copper is alloyed with silicon, leading to a reduction in charge/discharge capacity.
Japanese Laid-Open Patent Publication No. 2002-373644 (“Patent Document 2”) proposes forming an intermediate layer made of Mo or W on the surface of a current collector. The intermediate layer serves to prevent the constituent element of the current collector from excessively diffusing into an active material layer.
Patent Document 1 proposes controlling the temperature of the current collector at less than 300° C. during the formation of the silicon thin film, in order to prevent excessive diffusion of copper into the silicon thin film. However, since the diffusion coefficient of copper in silicon is high, it is difficult to suppress diffusion by merely controlling the temperature at less than 300° C. Also, in vapor deposition and sputtering, if the deposition speed is heightened to enhance production efficiency, the temperature of the copper foil rises. In order to control the temperature of the copper foil at less than 300° C., it is necessary to make the deposition speed low, which results in a reduction in production efficiency.
When a silicon thin film is formed on copper foil, the copper foil may curve due to the accumulation of internal stress in the silicon thin film. Such internal stress is relieved by heat-treating the obtained silicon thin film together with the copper foil. However, the heat treatment may cause excessive diffusion of copper into the silicon thin film.
Also, an effective method for compensating for the irreversible capacity loss of the negative electrode is deposition of lithium on the silicon thin film. However, when lithium is deposited, the temperature of the copper foil rises, so that copper may excessively diffuse into the silicon thin film.
As described above, with the method of Patent Document 1, it is difficult to control the diffusion of copper into the silicon thin film. Since a change in the amount of copper diffusion will cause a change in the charge/discharge capacity of the silicon thin film, it is difficult to obtain stable quality. Also, with the method of Patent Document 1, the interface between the copper foil and the silicon thin film becomes brittle, so the silicon thin film becomes separated during charge/discharge, thereby resulting in degradation of cycle characteristics.
Patent Document 2 proposes the formation of an intermediate layer comprising tungsten or molybdenum. Tungsten or molybdenum, however, does not serve as an active material. When a material that does not serve as an active material is used as the intermediate layer, the energy density of the negative electrode decreases. Also, since tungsten or molybdenum has a high melting point, it is difficult to increase the deposition speed. Hence, the formation of the intermediate layer requires large costs such as machine costs and operation costs.